A New Geometric-Distortion Solution for the STIS NUV- MAMA

We have obtained a new geometric-distortion solution for the STIS NUV-MAMA using two fields in NGC 4214 and NGC 604 which were imaged using three filters (CN182, CN270, SrF2) for GO program 9096. (WF)PC2 images of the same objects in F170W and F336W were used as an astrometric reference. Different polynomial degrees and weighting mechanisms for the geometric distortion were tested until an adequate solution was obtained. Typical median errors between the predicted and the real positions were found to be in the range of 0.28 to 0.53 MAMA pixels (7-13 mas). A new geometric distortion table has been produced and tested. We discuss differences between this solution and the previous one that was presented in STIS Instrument Science Report 2001-02. Introduction While developing a software tool for the analysis of objective-prism data obtained with the STIS NUV-MAMA, we discovered that the current implementation of the geometric distortion solution was incorrect. Apparently, the error originated in a different assumed shape for the output pixels between the solution developed by Walsh et al. (2001) and its implementation in the IDCTAB reference table. Walsh et al. (2001) assumed rectangular pixels (i.e. different x and y sizes) while IDCTAB assumed square ones (i.e. same size in x and y). As a result, the errors in the corrected positions are as large as ~5 pixels. The NUV-MAMA objective-prism data we had obtained consists of three different fields, one of them in the Giant H II Region NGC 604 and two in the nearby galaxy NGC Copyright© 1999 The Association of Universities for Research in Astronomy, Inc. All Rights Reserved. Instrument Science Report STIS 2004-01 4214. Each field was imaged using two different orientations, and each one of the orientations has two exposures with a small spatial displacement between them. The objectiveprism data was complemented with short imaging exposures using the same NUV-MAMA detector in order to find the positions of the sources in the main exposures. Here we will use some of those images for the purpose of deriving a new geometric distortion solution. The technique used by Walsh et al. (2001) involved taking multiple exposures of the same field with small displacements between them but with the same orientation and guide stars and involves no previous knowledge of the positions of the stars in the field. This method has the drawback that the skewing terms in the distortion solution cannot be constrained (Anderson & King 2003). Here we will use a different approach. One of the NGC 4214 fields has been imaged with the PC of the WFPC2 using F170W and F336W using two orientations which differ by almost 180o from one another (see Fig.1) while the NGC 604 field has been imaged with the PC using F170W and a single orientation. Given that we had data in the same wavelength range obtained with an instrument with an accurate geometric distortion solution (Anderson & King 2003, Kozhurina-Platais et al. 2003), we decided to measure the positions of the stars in the PC images, correct the geometric distortion there, and use those ‘real’ positions to generate an ‘astrometric flat field’. Then the positions of the same stars could be measured in the NUV-MAMA images and the geometric-distortion solution calculated directly using the difference between the measured and the real positions. This is the same idea which was used recently by KozhurinaPlatais et al. (2003) to produce a multi-wavelength geometric distortion solution for the WFPC2. Those authors used F555W positions to generate the astrometric flat field and then measured the positions of the same stars in other filters. Data The STIS and WFPC2 data sets used in this work are listed in Tables 1 and 2 respectively. All STIS datasets were obtained for GO program 9096 (P.I.: Maíz-Apellániz). The NGC 4214 WFPC2 data sets were obtained for GO program 6716 (P.I.: Stecher) and the NGC 604 WFPC2 data sets were obtained for GO program 5384 (P.I.: Waller). Table 1. STIS data sets used. field/orientation CN182 CN270 SrF2 NGC4214/1 o6bz02isq, iwq o6bz02j7q, jbq NGC 4214/2 o6bz01afq NGC 604/1 o6bz06wuq NGC 604/2 o6bz05p7q